A safety system is known, for example, from the essay by W. Suchowerskyj “Evolution en matiére de détecteurs de choc” [Developments in impact detectors] in Ingénieurs de l'Automobile 1141 (1982) No. 6, pp. 69-77, Paris. The safety system encompasses a sensor suite having a plurality of sensors, at least one control unit, and a plurality of restraint means, for example airbags and/or belt tensioners. The sensor suite of a safety system of this kind serves to sense and evaluate an accident situation. On the basis of the data obtained, the points in time for triggering of the appropriate restraint means are determined.
Acceleration sensors, but also rotation-rate sensors and pressure sensors, are common today. They serve to measure acceleration, rotation rate, or an intrusion into the vehicle in the various axes, and thus to detect the progression and severity of an accident. The sensors can be disposed in a central control unit or also as external components in the front region or the side regions of the vehicle. As with all components of a safety systems, monitoring of these sensors is of critical importance in terms of the reliability and availability of the entire safety system. The safety system can provide its intended protective effect only if fault-free operation of the sensor suite is ensured.
The external sensors are usually connected via a two-wire conductor to a control unit. The conductor supplies the sensors with electrical energy from the control unit, and also serves to transfer digital data from the sensors to the control unit. For that purpose, the data are transmitted from the sensors, preferably serially, as current values. The monitoring of such external sensors is of particular importance because short circuits or interruptions are always a possibility in the case of a wiring harness routed in a vehicle. The monitoring system must on the one hand protect the electronic components, i.e. in particular in the context of short circuits to ground or to battery voltage. On the other hand, it is important specifically in the case of external components to detect the actual cause of a fault as accurately as possible, so that a repair can be quickly and cost-effectively performed in the shop.
With the monitoring that is common today, an analog measurement of the voltage on the supply conductor is performed for this purpose. Short circuits to ground or to the battery can be detected thereby because the potential then deviates from the normal supply voltage of the external sensors (e.g. 6 V). If the external sensor is sending no data, however, it is impossible to state unequivocally whether this is caused by a conductor interruption or because of an internal defect in the sensor. With the method hitherto usual, i.e. measurement of the voltage on the connecting conductor to the external sensors, it is possible to detect certain fault patterns. If the peripheral sensor is no longer sending data, a diagnosis is made according to the following formula: Measure the voltage on the connecting conductor. If it is distinctly lower than the nominal supply level to the sensors, this suggest a short circuit to ground. If it is above the threshold, however, the power supply to the sensor is switched off and another measurement is performed. The connecting conductor should now be at zero potential. If a voltage is nonetheless measured, this suggests a short circuit to the battery. If no voltage is measured, however, a conductor interruption or a sensor defect is deduced as the cause of the sensor fault.
This type of diagnosis thus makes it possible to classify essentially three fault patterns: short circuit to ground, short circuit to battery, conductor interruption or sensor with internal defect. While the first two fault types are unequivocally attributable to the vehicle's wiring harness, this is not immediately possible in the case of the third fault category. If this fault is recorded in the fault memory, testing must be performed in the shop to discover whether the fault is to looked for in the vehicle's wiring harness or in the sensor itself. This, however, is time-consuming and expensive.